Method for forming hollow articles from thermoplastic sheeting or film using an expandable plug

ABSTRACT

A method for using an expandable forming plug in forming apparatus such as plug assisted vacuum or pressure differential forming machines and the like wherein the plug cooperates with a mold to form hollow articles such as containers, closures, trays, and the like, from thermoplastic synthetic resinous film or sheeting. The expandable plug provides hollow articles which during forming can be programed or controlled, as desired, with respect to thickness distribution in the walls thereof and, more particularly, provides hollow articles with side and bottom walls of desired thickness, such as of substantially uniform or of a varying thickness.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division, of application Ser. No. 421,005 filed Dec. 3, 1973,now issued as Pat. No. 3,901,640.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

This invention relates to new and useful improvements for making hollowarticles such as containers, closures, trays, and the like, fromthermoplastic synthetic resinous film or sheeting and, moreparticularly, to a method for using an expandable plug or plunger whichcooperates with a forming mold in forming apparatus to provide hollowarticles which during forming can be programed or controlled, asdesired, with respect to thickness distribution throughout the wallsthereof.

2. Description of the Prior Art:

At the present time, a variety of hollow articles are formed fromthermoplastic synthetic resinous materials. They include containers,closures for containers, trays and the like which are generally formedby injection molding or mechanical drawing processes.

In a mechanical drawing process, plastic sheeting or film may beexcessively stretched thereby resulting in the formation of hollowarticles having an objectionable variable wall thickness, particularlyin deep corner areas, or an objectionable appearance due to visible flowlines in the walls thereof. These problems are caused by the sheeting orfilm sticking to the forming mold thereby producing thick wall sectionswhere the mold first contacts the sheeting or film and excessively thinwall sections in deep drawn corner areas. In extreme cases, hollowarticles will have deep drawn areas with very thin wall thicknesseswhich reduce their marketability because of inadequate strength, impactresistance, ruggedness and/or unpleasant appearance. Furthermore,relatively thick sheeting or film may be necessary to overcome theseproblems, thereby producing hollow articles which cannot economicallycompete with similar articles formed from other structural materialssuch as metal.

To overcome the above problems, a plug or plunger is often used toassist in the formation of mechanically drawn hollow articles. Forexample, a plug or plunger is often used to push heat softened plasticsheeting or film into a female forming mold of a vacuum forming machine.While the general utilization of plugs or plungers in cooperation withforming molds may improve the formation of hollow articles, the knownplug or plunger designs have not been entirely satisfactory ineradicating the material distribution problems in the walls of deepdrawn hollow articles and, in some instances, have created otherproblems such as plug mark off and contamination of the formed articles.In this regard, the previously discussed sticking of the sheeting orfilm to a mold may also occur on a plug or plunger causing the formationof hollow articles with excessively thick bottom walls and thinned outdeep corner areas.

Of the known types of plugs or plungers, the most commonly used is asolid metal or wooden plug which is somewhat smaller than andapproximately the same shape as the forming cavity in the mold. Thistype of plug is illustrated in U.S. Pat. No. 2,891,280. Other knownplugs which attempt to improve material distribution in the walls ofhollow articles by deforming during processing are disclosed in U.S.Pat. Nos. 2,966,873 and Re 24,833. Plugs with felt or fabric covers andplugs which are lubricated have also been used to improve slip of heatsoftened plastic sheeting or film over the plug thereby providing betterwall thickness distribution in the resulting hollow articles formedtherewith.

SUMMARY

In general, the present invention relates to a method for using anexpandable forming plug or plunger in apparatus to produce hollowarticles from thermoplastic synthetic resinous sheeting or film ofsubstantially uniform thickness. The expandable plug is adapted for usein cooperation with forming molds of single or multiple cavity formingapparatus such as vacuum or differential pressure thermoforming machinesand the like to form hollow articles such as containers, closures forcontainers, trays, and the like. The expandable plug provides improvedhollow articles which during forming can be programed or controlled, asdesired, with regard to thickness distribution in the walls thereof and,more particularly, provides thermoformed hollow articles with side andbottom walls of substantially uniform or of a controlled variedthickness. For instance, in a well-controlled process, it is possible touse an expandable plug which will produce a hollow article having a wallthickness in deep corner areas equal to or greater than that obtainablein other portions of the side or bottom walls. The expandable plug canbe made from many materials including metals such as steel or aluminum,wood, plastics such as nylon, rubber and the like.

The expandable plug or plunger, adapted for use in cooperation withforming molds in apparatus for producing hollow articles fromthermoplastic synthetic resinous sheeting or film, comprises a pluralityof adjacent elongated segments, means for holding one end of thesegments in a juxtaposed pivotal relationship with each other, means formoving the other opposing ends of the segments outwardly away from oneanother as the segments advance and force plastic sheeting or film intoa forming mold and means for returning the other ends of the segments totheir original adjacent position as the same are withdrawn from theforming mold.

The expandable plug may also include a flexible, resilient cover orsleeve positioned around the plurality of segments to reduce mark off orstriation in the hollow articles being formed with the expandable plug.The cover or sleeve can be made from any flexible, resilient material,but is preferably made from a material such as silicone rubber which hasgood wear resistance during prolonged high temperature use. In addition,means for heating and/or cooling may also be included in the expandableplug thereby improving control of material distribution in hollowarticles formed with the expandable plug.

Accordingly, an object of the present invention is to provide a methodfor using an expandable plug or plunger in forming apparatus to producehollow articles from thermoplastic synthetic resinous sheeting or film.

A further object is to provide a method for using an expandable plug orplunger in forming apparatus such as vacuum and/or pressure differentialforming machines to form heated thermoplastic synthetic resinoussheeting or film into hollow articles such as containers, closures forcontainers, trays, and the like.

A still further object is to provide a method for using an expandableplug or plunger which cooperates with forming molds in forming apparatusto produce hollow articles that, during the formation thereof, can beprogramed or controlled, as desired, with respect to thicknessdistribution in the walls thereof and, more particularly, to producehollow articles with side and bottom walls of substantially uniform orof a controlled varied thickness.

Additional objects and advantages of the present invention are even moreapparent when taken in conjunction with the accompanying specification,drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further understood by reference to the accompanyingdrawings in which like characters of reference designate correspondingmaterials and parts throughout the several views thereof, in which:

FIG. 1 is a sectional view of an expandable plug constructed accordingto the principles of the present invention;

FIG. 2 is a sectional view of the expandable plug illustrated in FIG. 1showing the plug in its expanded position;

FIG. 3 is a sectional view illustrating a modified expandable plugconstructed according to the principles of the present invention;

FIG. 4 is a schematic illustration of the operation of the expandableplug shown in FIGS. 1 and 2 as compared to the operation of a standardsolid metal plug; and

FIGS. 5A and 5B are partial sectional views of a common plasticcontainer or tub and a graph comparing the thickness distribution in thewalls of containers or tub formed with a standard solid metal plug andthe expandable plug shown in FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description illustrates the manner in which the principlesof the invention are applied but are not to be construed as limiting thescope of the invention.

More specifically referring to FIGS. 1 and 2, an expandable plug orplunger 10 in its unexpanded and expanded positions, respectively, isillustrated. Plug or plunger 10 is designed for forming round, straightor slanted side wall hollow articles, although plugs or plungers similarin design to plug 10 can also be constructed to produce different shapessuch as oval, square or rectangular hollow articles and the like. Plugor plunger 10 is adapted for use in cooperation with female formingmolds, not shown in FIGS. 1 and 2, of single or multi-cavity apparatusused to produce hollow articles from thermoplastic synthetic resinoussheeting or film. Plug 10 can be made from any desired structuralmaterial, but is preferably made from a metal such as aluminum or steel.

Expandable plug 10 comprises a plurality of closely spaced adjacentsegments or fingers 12 held in juxtaposed pivotal relationship with oneanother at the base end 14 thereof by a base plate 16 and ring clamp 18.Base plate 16 includes fastening means 20 for attaching plug 10 into theforming station of any common forming apparatus, not shown, used to formhollow articles. Plug 10 is positioned in a similar manner as the normalsolid plug used in such forming apparatus. Plug 10 also includes a cam22, rod 24, spring 26, adjustable stop nuts 28 and a flexible, resilientcover or sleeve 30 formed of silicone rubber positioned around fingers12. Cover or sleeve 30 includes a band portion 32 for holding cover 30in a fixed position on fingers 12 and to insure that fingers 12 are heldin a juxtaposed relationship before expansion of the same. Cover orsleeve 30 also includes a concave portion 34 inside the forward ends 36of fingers 12 to prevent plastic film or sheeting from sticking orfreezing to this area during the forming operation.

During operation of forming apparatus incorporating plug 10, cam 22interacts with camming surfaces 38 of fingers 12 to expand the forwardends 36 of fingers 12. As plug 10 is advanced, forcing heated plasticsheeting or film into a forming mold, not shown, cam 22 is activatedafter plug 10 has advanced a predetermined distance into the formingmold, preferably after forward ends 36 have reached at least about threequarters of the depth of the form mold. Activation of cam 22 isaccomplished by a simple mechanical stop, not shown, in the formingapparatus which prevents the continued movement of rod 24 after plug 10has advanced the desired distance into the forming mold. When plug 10 isretracted from the forming mold, spring 26, band portion 32 and stopnuts 28 cooperate to return fingers 12 to their original unexpandedjuxtaposed position. Adjustable stop nuts 28 may also be used to effecta degree of variability in the original juxtaposed position of fingers12 thereby increasing control over material distribution in hollowarticles formed with plug 10.

Referring now to FIG. 3, a modified expandable plug 40 is illustrated.Plug 40 comprises fingers 42 having base ends 44 and forward ends 66,base plate 46, ring clamp 48, fastening means 50, cam 52, rod 54, spring56, adjustable stop nuts 58 and cover or sleeve 60 having a band portion62 and concave portion 64, all of which are structurally andfunctionally similar to corresponding parts of plug 10 with theexception of the camming mechanism. Cam 52 must be moved forward by rod54 to expand forward ends 66 of fingers 42. This camming mechanism isparticularly suited where plug 40 is held in a fixed position in aforming apparatus and the forming mold is moved to the same during theforming operation.

It has been found that the flexible resilient covers 30 and 60 of plugs10 and 40, though preferred to prevent mark off or striation in hollowarticles as a result of plastic sheeting or film sticking to the forwardends 36 and 66 of fingers 12 and 42 and then thinning out betweenfingers 12 and 42 as they are expanded, are not necessary if a largenumber, preferably about 200 to 300, of fingers 12 and 42 are used.However, as a practical matter, flexible covers 30 and 60 are preferredbecause the use of a large number of fingers 12 and 42 presentsdifficult assembly and maintenance problems and because the elasticbehavior of covers 30 and 60 positioned over fingers 12 and 42 providesexcellent radial thinning and material distribution of the plasticsheeting or film at the forward points of contact with plugs 10 and 40as the same are expanded. When an expandable plug is used without covers30 and 60, the band portions 32 and 62 of the same can be replaced by acommon flexible rubber "O" ring.

Plugs 10 and 40 may also include heating and/or cooling means, notshown, since it is known that the temperature of a plug used in formingapparatus is a significant variable for controlling materialdistribution in hollow articles during formation thereof. However, ithas been found that the expandable plugs herein described are much lesscritical or sensitive with respect to temperature control thereof thanare other known solid plug designs. Furthermore, the use of heatingand/or cooling means to control the temperature of expandable plugs thatdo not include covers 30 and 60 will help prevent the mark off orstriation problem previously discussed.

To better illustrate the comparative differences between the use ofsolid metal plugs and the expandable plugs, herein described, in formingapparatus such as a vacuum thermoforming machine, reference is now madeto FIG. 4. Specifically, FIG. 4 schematically illustrates four steps,a-d and a'-d', respectively, in the formation of a container from heatedpolystyrene sheeting 74 with a solid metal plug 72 and expandable plug10 used in cooperation with a vacuum forming mold 70. As noted in stepsa and b, heated sheeting 74 sticks to bottom and lower side walls ofsolid plug 72 as soon as it contacts the same. This results in a thinbottom corner area 76 at step d since it is obvious from step c thatthere is a limited amount of sheeting 74 available to be drawn intocorner area 76. When expandable plug 10 is used, it is shown in step a'and b' at points 78 and 79 that increased pressure between sheeting 74and mold 70 occurs as a result of the expansion of plug 10 whichprovides better material distribution of sheeting 74. In addition, it isshown in steps b' and c' how, by use of the expandable plug of thisinvention, sheeting 74 from the bottom area of plug 10 is mechanicallydrawn into corner areas 80 resulting in better distribution of materialbetween container bottom and corner areas 80.

Referring to FIGS. 5A and 5B, a further illustration of a round, slantedside wall container 90, such as may be used to package dairy products,formed with a solid metal plug and an expandable plug described herein,and a graph comparing the side and bottom wall thickness distribution atvarious points, is shown. Containers 90 were formed on a vacuumthermoforming machine using standard forming conditions from heatedpolystyrene sheeting using an expandable plug like plug 10 and a commonsolid metal plug. The containers 90 were then cut in half and measuredfor thickness at points 1-20, shown in FIG. 5A. The thicknessmeasurements were then plated on a graph, shown in FIG. 5B, with heavyline 92 representing the wall thickness distribution of container 90formed with the expandable plug and light line 94 representing the wallthickness distribution of container 90 formed with the solid metal plug.Lines 92 and 94 clearly illustrate the typical contour and thicknessdistributional differences of the expandable plug and solid metal plug.Of particular interest is the difference found at point 12, the bottomoutside corner of container 90. The expandable plug forms a thicker wallsection at this point where the majority of severe impacts during use ofcontainer 90 can be expected. As a result, a container having an overallthinner wall thickness and lighter weight can be made with an expandableplug which will have the impact resistance of a thicker and heaviercontainer formed with a solid metal plug. Similarly, it is possible touse an expandable plug of this invention to provide articles havingvariable programed wall thickness, whenever such may be desirable.

While certain representative embodiments in detail have been shown forthe purpose of illustrating the invention, it will be apparent to thoseskilled in the art that various changes and modifications can be madewithout departing from the spirit and scope of the invention. Forexample, the scope of the present invention is not limited to anexpandable plug designed to only forming round, slanted side wall hollowarticles, as illustrated in FIG. 4, since an expandable plug can bedesigned to form straight side wall hollow articles, and oval, square orrectangular hollow articles and the like.

What is claimed is:
 1. A method of forming a hollow article fromthermoplastic synthetic resinous sheeting or film, comprising:a.providing an expandable plug and forming mold in an apparatus adaptedfor forming said hollow article, said expandable plug comprising aplurality of adjacent elongated pivotal segments, means for holding oneend of said segments in a juxtaposed pivotal relationship with eachother, means for moving the other opposing ends of said segmentsoutwardly away from one another as said segments advance and force saidsheeting or film into said forming mold, means for returning saidsegments to their original adjacent position as said segments arewithdrawn from said forming mold and a flexible, resilient cover orsleeve positioned around said plurality of segments; b. forcing saidsheeting or film into said forming mold with said expandable plug in itsoriginal unexpanded position; c. moving the forward ends of the pivotalsegments of said expandable plug outwardly away from one another as saidplug moves into said forming mold to draw the sheeting or film from thebottom area of the plug into corner areas of the mold and also thin thesidewall portions thereof whereby the elastic behavior of said cover orsleeve provides radial thinning and controlled material distribution ofthe sheeting or film upon forming said hollow article in said mold; d.returning said expandable plug to its original unexpanded positionoutside of said forming mold after said hollow article has been formed;and e. removing said formed hollow article from said mold.
 2. The methodof claim 1 wherein said expanding step is accomplished after said plughas advanced a predetermined distance into said forming mold.
 3. Themethod of claim 2 wherein said predetermined distance is at least aboutthree quarters of the depth of said forming mold.